Look in a mirror and what do you see? Your mirror image, only everthing is reversed from the part of your hair to the label on your shirt. For the Supermen, their mirror images were Bizarro of Earth-One and Flying Tiger of Earth-Two, each being made of non-living matter designed to replicate their template's powers. In the process, they also gained their opposite number's weaknesses, specially Kryptonite. Superman used the imperfect duplicator ray to make blue Kryptonite from the original green material His counterpart didn't need to go that far, instead using the real thing versus Flying Tiger since the bio-ray projector wouldn't have worked on the substance. The end process was the same, as such Kryptonite proved deadly (as much as anything can towards imitation life) to Bizarro and the Tiger.
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